1. Field of the Invention
This invention relates to a technically simplified process for the production of polyurethane urea elastomers in which high-melting aromatic diamines present in solid phase are reacted with polyisocyanates and isocyanate prepolymers respectively in a heterogeneous reaction.
2. Description of the Prior Art
The production of polyurethane urea elastomers from polyisocyanates, relatively high molecular weight polyhydroxyl compounds and aromatic diamines is known. To guarantee reasonable processing times for reactive systems of the starting components mentioned, the most widely used reactive aromatic isocyanates are preferably reacted with sluggishly reacting diamines. The diamines which have been successfully used in practice for this purpose are, above all, aromatic diamines wherein the basicity and, hence, reactivity to isocyanates has been reduced by introduction of halogen or carboxy substituents. 3,3'-dichloro-4,4'-diaminodiphenylmethane (MOCA), previously the most widely used aromatic diamine, is mentioned as an example.
US-PS 3 891 606 describes the crosslinking of NCO prepolymers (prepared from polyhydroxyl compounds and an excess of polyisocyanates) with aromatic diamines whose reactivity to isocyanate groups has been reduced by complexing with certain alkali metal salts. The disadvantage of this process lies in the fact that it is confined to two particular aromatic diamines. In addition, the complex between the aromatic diamine and the alkali metal salt has to be prepared in a separate process step.
Another way of controlling the reaction rate between polyisocyanates and aromatic diamines is to carry out the reaction in an organic solvent. Processes are described, for example, in US-PS 3 926 922 and in published Japanese patent application 9195/70. The disadvantage of using organic solvents is obvious. First, the risk of fire and explosions is increased; second, technically elaborate recovery of the solvent is necessary for both economic and ecological reasons.
Previously, little has been known of the production of polyurethane ureas by reaction of polyisocyanates with aromatic diamines in a heterogeneous phase. According to the prior art, the relatively high-melting aromatic diamines of particular technical interest are either used in dissolved form, which involves the disadvantages described, or are reacted with polyisocyanates in the melt. The processing of aromatic diamines in the melt is described, for example, in the above-cited US-PS 3 926 922 and in DE-AS 1 122 699. DE-AS 1 122 699 relates to a process for the production of polyurethane elastomers by crosslinking liquid isocyanate prepolymers with mixtures of primary diamines and compounds containing several hydroxyl groups in molds. A dispersion of a powder-form crystalline diamine in a liquid polyester or polyether containing several hydroxyl groups or in castor oil is introduced into the prepolymer at a temperature below the melting point of the diamine and the resulting mass is hardened in known manner at temperatures above the melting point of the diamine in the mixture. In this process, therefore, the actual "amine crosslinking" again takes place in a liquid homogeneous phase. The disadvantage of the process according to DE-AS 1 122 699 lies in particular in the high temperatures required for the processing of high-melting diamines such as 1,5-naphthylenediamine (Mp. 189.degree. C.) or 4,4'-diaminodiphenyl ether (Mp. 186.degree. C.) because experience has shown that decomposition reactions occur to a considerable extent in the polyurethane at these temperatures, uncontrollably altering the mechanical properties of the end products.
US-PS 3 105 062 describes a process for the production of polyurethane ureas in which relatively high molecular weight preadducts containing isocyanate groups are preferably reacted with aromatic diamines in a heterogeneous phase. The reaction mixture hardens at a temperature at which the "two-phase system" changes into a "one-phase system." This temperature is generally in the range of 100.degree. to 170.degree. C. However, the aromatic diamines mentioned in US-PS 3 105 062 are soluble, albeit to a limited extent, in the reaction medium (NCO preadduct) so that uncontrollable preliminary reactions occur during the mixing of the two components, even at room temperature. The result is that the reaction mixtures thicken up in a very short time, paste-like formulations being obtained in some cases. Such formulations are difficult to process by the usual casting technique and, accordingly, have to be brought into the required shape by application of pressure before they are actually hardened by the application of heat. It is disclosed in US-PS 3 105 062 that the stability of the thickened reaction mixtures in storage (pot life) is sufficient for further processing (molding under pressure, coating), amounting to several hours. However, it can be seen from the examples that the reaction mixtures in question are preferably reaction mixtures which have a maximum pot life of about 1 hour. Accordingly, they cannot be regarded as long-term systems.
In US-PS 3 105 062, it is specifically pointed out that the use of the diamines mentioned, which are present in solid form only, in a one-shot process gives unsatisfactory polyurethane moldings. In this case, the unwanted preliminary reaction of the diamine with the diisocyanate takes place to a greater extent such that the poorly soluble polyurea precipitates in the reaction mixture and ceases to react.
DE-OS 26 35 400 describes another process for the production of polyurethane urea elastomers, in which aromatic diamines are reacted as chain-extending agents in a one-shot or multistep process. This process is characterized in that the aromatic diamines are present in solid form in the reaction mixtures and have a melting point above 130.degree. C. The thermosetting of these mixtures takes place at a temperature in the range of 80.degree. to 120.degree. C., i.e., below the melting point of the aromatic diamine. By virtue of the choice of the corresponding diamines as chain-extending agents, there is no mixture-thickening preliminary reaction with the NCO preadduct (NCO prepolymer). Accordingly, systems such as these may also be readily processed by casting. Since the pot life of these reactive systems is considerably increased, it is possible in this process to use many aromatic diamines which have always been very difficult to process by previously known techniques.
It can be seen from the examples of DE-OS 26 35 400 that the pot life of the liquid reaction mixtures varies from a few minutes to several hours in a certain temperature range, depending on the reactivity or solubility of the aromatic diamine. Under standard processing conditions, for example in hand casting, these reaction mixtures, particularly when they have relatively long pot lives, can generally be processed without major difficulties. By contrast, processing becomes difficult when, as a result of machine stoppages or other enforced interruptions, there is a relatively long interval between preparation of the reaction mixtures and the hardening phase. Accordingly, the need for a long processing time at low temperature and for a short setting time at elevated temperature is increasingly important in practice.
The final PUR plastics are generally required to show good mechanical properties and, in many cases, a thermal stability adapted to the application envisaged. In the prior art, the thermal stability of PUR elastomers depends largely on the type of chain-extending agent used. When glycolic chain extenders for example are used for the production of elastomers, the PUR moldings obtained show lower thermal stability than when compounds containing amino groups are used. There are of course also distinct differences with regard to thermal stability within the particular type of chain-extending agents, i.e., either compounds containing OH or NH groups.
Many types of diamines are mentioned in DE-OS 26 35 400 as suitable chain extenders for the production of polyurethane urea elastomers. However, 2,2'-diaminodiphenylurea is the only disclosed representative of a diaminodiphenylurea and no example is provided.
Investigation has shown that the reaction of an NCO preadduct with 2,2'-diaminodiphenylurea gives an elastic PUR molding characterized by entirely acceptable mechanical properties. However, the thermal stability of these elastomers is unexpectedly poor. Even when the molding is conditioned at 120.degree. to 130.degree. C., there is a significant reduction in its mechanical properties after only a short time. At 140.degree. to 150.degree. C., only a viscous melt is present, irrespective of whether the sample is hot or cold.
Accordingly, the object of the present invention is to provide a process for the production of polyurethane ureas in which the processing time of the reaction mixtures at the particular processing temperature is at least several hours and preferably at least 8 hours (long pot life). It is also desirable for the liquid reaction mixtures to set in economically acceptable times at low reaction temperatures.
Another object of the invention is to provide suitable chain-extending agents containing amino groups which show only minimal solubility in the starting component (e.g. NCO preadduct) at low temperature, but very good solubility therein at elevated temperature, so that the polyurethane urea is synthesized with as high a molecular weight as possible during the hardening phase of the mixtures.
A further object of the present invention is to provide a process for the production of polyurethane urea elastomers in which high-quality elastomers of high heat resistance are obtained.
It has now been found that these objectives may be achieved in accordance with the present invention.